Organic electroluminescent devices, which operate by electroluminescence (hereinafter abbreviated as EL) from organic materials, (what are called organic EL devices), are completely solid devices of a thin film type capable of emitting light at a low voltage ranging from several V to several 10 V and have many good characteristics such as high luminance, high emission efficiency, small thickness, and lightweight. In recent years, therefore, they have attracted attention as backlights for a variety of displays, display boards for signboards, emergency lights, and the like, and surface emitting devices for illumination light sources, etc.
Such organic electroluminescent devices have a structure including two electrodes and a light-emitting layer including an organic material disposed between the electrodes, in which light produced by the light-emitting layer is extracted through the electrode. Therefore, at least one of the two electrodes is formed as a transparent electrode.
Transparent electrodes generally used include oxide semiconductor materials such as ITO (indium tin oxide, SnO2—In2O3). A stack of ITO and silver layers is also studied to provide lower resistance (e.g., refer to Patent Literatures 1 and 2 listed below). Unfortunately, ITO, which contains a rare metal, indium, is a high-cost material and needs to be annealed at about 300° C. for resistance reduction after it is deposited as a film. Therefore, there are proposed a thin film of a high-conductivity metal material such as silver and a film of a mixture of silver and aluminum, which can ensure conductivity with a thickness smaller than that of a film of silver alone (e.g., refer to Patent Literature 3 listed below). There is also proposed a multilayer structure including an underlying layer of a metal other than silver and a silver thin-film layer provided on the underlying layer so that optical transparency can be ensured (e.g., refer to Patent Literature 4 listed below).